There are certain qualities and mannerisms about gay men that quite a lot of them do have. We can be as progressive as we want and say that being interested in other people of the same sex is the only hint (and yes, it is the only sure-fire 100% accurate one) but the fact remains that the stereotypes came from somewhere and it's easy to see if you're tuned into it.

I'm not an effeminate person and quite a lot of people were shocked when I came out because they had no idea, but there were also people that weren't surprised and had picked up on it, like my aunt and uncle and my best friend. Those people have good gaydar, the others do not.